Prospect House, George Cayley Drive, Clifton Moor, York, England YO30 4XE
Tel: +44 (0) 1904 692723 Fax: +44 (0) 1904 690385
E-Mail: [email protected] www.yorksurvey.co.uk
Suunto Tandem
Combination
Compass/Clinometer
Code: 36027
Operating
Instructions
Ref:.. \operat98\instructions 15\36027.qxp 05-01-15
©York Survey Supply Centre 2015
All readings on the percentage scale are based on the horizontal
distance. This means that if the distance on sloping terrain is measured
along the ground, an error is introduced and this must be corrected for
accurate results. The error is insignificant for most purposes at small
ground slope angles but increases progressively as the angle
increases.
The trigonometrical correlation is H = h x cos a where
H = the true or corrected height
h = the observed height
a = the ground angle
With the aid of the above equation, the correction can also be made in
the distance where
h = the distance measured along the ground
H = the horizontal distance sought. If the corrected distance is used, no
correction in the height observed is needed.
When calculating the horizontal distance by using the ground distance
and slope angle, it must be pointed out that an error is introduced if the
slope is measured from eye level to the pillar base. Measuring the
slope along the ground would be cumbersome and inconvenient. No
error is introduced, however, when the slope angle is measured from
eye level to a sighting mark made or placed on the pillar at eye level
(Fig. 11) whereby the two lines of measurement become parallel. the
true angle of slope is 9 degrees. The example shown in Fig. 11
illustrates both methods of calculation.
Fig.11. Calculating horizontal distance by using ground distance and
slope angle
Method 1: Measure the ground distance. This is found to be 25m (82ft).
Then measure the slope angle. This is 9°. Read percentages of top and
ground points. These are 29 and 23%.
Calculate:
23 29 52
100 + 100 = 100
Take 52% of 25m (82ft). This is 13m (42.6ft). Multiply this by the cosine
of 9°.
0.987 x 13m = 12.8m (0.987 x 42.6ft = 42ft)
Method 2: Multiply the ground distance by the slope angle cosine
(straight distance). 0.987 x 25m = 24.7m (0.987 x 82ft = 80.9ft). Add
percentage readings as above and take the sum percentage of the
corrected distance. 52 / 100 x 24.7m = 12.8m (52 / 100 x 80.9ft = 42ft).
This example shows that a slope angle of 9° causes a correction of
only 2.3% but when the slope angle is 35 degrees, the correction
means a reduction of about 18% in the observed height.
Nomographic Height Correction
When the accompanying nomogram is used, all correction calculations
become unnecessary. Only a ruler or some other convenient object
with a straight edge is needed to obtain the nomographical solution.
The nomogram is used by placing the ruler so that its edge intersects
the angle scale on the left at the slope angle point and the observed
height scale (on the right) at the pertinent point. The corrected height
(or distance) is read at the point where the edge intersects the
corrected height scale in the middle. When using a measuring distance
of 20m or 100ft along the ground, the correction procedure becomes
very simple. No slope angle measurement is then necessary. One
needs only the reading of the top point and that of the ground point.
Depending on the situation, their sum or difference gives the apparent
height directly in feet. This is then corrected as follows:
First, find on the right-hand scale in the nomogram the point indicating
the apparent height. Secondly, find on the left-hand double scale the
point indicating the ground point reading. Thirdly, connect these points.
The corrected reading will be found from the pertinent middle scale at
the point of intersection. In this procedure, the slope angle can be
neglected as the left-hand ground point scale has been constructed so
that both the ground slope angle and the average eye level height of
1.6m (5.5ft) have been taken into account.
Fig. 12.
ISO 9001
SUUNTO Oy’s Quality Assurance System is certified by Det Norske
Veritas to be according to the ISO 9001 in all SUUNTO Oy’s operations
(Quality Certificate No. 96-HEL-AQ-220).
